In the fields of heat-resistant coating materials, electrical insulating materials such as interlayer insulating materials of printed wiring boards and insulating materials of semiconductors, and electrical and electronic industry covering build-up materials, resins for prepreg, and heat-resistant adhesives, a resin composition that can provide a cured product having high toughness, high heat resistance, and high flame retardancy achieved without halogens has been demanded. In particular, in the field of electronics industry covering computers and the like, downsizing such as a significant decrease in the thickness of substrates, e.g., flexible film substrates and rigid substrates, has been highly demanded. To satisfy such a demand, it is essential to improve the mechanical strength (toughness), heat resistance, dimensional stability, flame retardancy achieved without halogens, and resistance to pyrolysis at high temperature, of a protective layer, adhesive layer, and insulating layer of substrates.
Polyimide resins have high heat resistance and mechanical strength and thus are favorably used in the field of electrical and electronic industry. For example, a polyimide obtained by causing a reaction between pyromellitic acid anhydride and 4,4′-bis(4-aminobenzamide)-3,3′-dihydroxybiphenyl to obtain polyamic acid and then subjecting the polyamic acid to ring-opening through dehydration has been disclosed as a polyimide resin having high heat resistance, mechanical strength, and dimensional stability (e.g., refer to PTL 1). However, the polyimide resin disclosed in PTL 1 has insufficient solubility in a solvent. Thus, the polyimide resin has limited uses.
Therefore, a solvent-soluble polyimide resin having high solubility in a solvent has been actively studied in recent years. For example, an imide resin mainly composed of tetracarboxylic acid anhydride having a structure in which ester bonds are formed at both terminals of an alkylene in the form of an acid anhydride and a diamine compound having an oxyalkylene structure has been disclosed as a polyimide resin that has high solubility in a solvent and dissolution stability and that provides a flexible cured film (e.g., refer to PTL 2). However, since the imide resin disclosed in PTL 2 has an ester bond and an aliphatic ether structure, sufficient heat resistance, dimensional stability, flame retardancy, and resistance to pyrolysis at high temperature cannot be achieved.
Furthermore, a polyimide resin whose solubility in a solvent is improved by using, for example, an aliphatic and/or alicyclic component such as sebacic acid or cyclohexanedimethanol as a constituent component has been disclosed (e.g., refer to PTL 3). However, the polyimide resin disclosed in PTL 3 has an aliphatic structure or an alicyclic structure. Since excessively high solubility in a solvent is pursued, sufficient heat resistance, dimensional stability, flame retardancy, and resistance to pyrolysis at high temperature cannot be achieved.
Moreover, a polyimide resin having a carboxyl group, a linear hydrocarbon structure, a urethane bond, and an isocyanurate structure has been disclosed as a polyimide resin that is soluble in typical organic solvents (e.g., refer to PTL 4). The polyimide resin disclosed in PTL 4 is soluble in a solvent other than N-methylpyrrolidone, but the polyimide resin alone provides poor film-formation property and thus an epoxy resin needs to be used together. A cured film obtained by using an epoxy resin together has heat resistance, but insufficient dimensional stability, mechanical properties such as toughness, and resistance to pyrolysis at high temperature.
As described above, since efforts have been excessively concentrated on an improvement in the solubility in a solvent in recent years, a polyimide resin having high dimensional stability is currently not provided.